Further notes from Wikipedia

By 864, the area around Durnovaria/Durngueir was dominated by the Saxons who referred to themselves as Dorsaetas, 'People of the Dor' - Durnovaria. The town became known as Dornwaraceaster or Dornwaracester, combining the original name Dor/Dorn from the Latin and Celtic languages with cester, Old English for walled town. This changed over time to Dorncester/Dornceaster and Dorchester. The town was a thriving commercial and political centre for south Dorset with a textile trading and manufacturing industry which continued until the 17th century.

In 1833, the Tolpuddle Martyrs founded the Friendly Society of Agricultural Labourers. Trade unions were legal but because the members swore an oath of allegiance [to the union], they were arrested and tried in the Shire Hall which is preserved as it was at the time. Beneath the courtroom are cells where the prisoners were held while waiting trial. Dorchester Prison was constructed in the town during the 19th century and was used for holding convicted and remanded inmates from the local courts until it closed in December 2013.